1. Field of the Invention
The present invention relates to a detector for demodulating a received signal and producing an information data signal with reduced intersymbol interference, and more particularily to a detector for demodulating a trellis coded modulation received signal.
2. Brief Description of the Related Art
To meet the ever increasing demand for high speed, reliable data transmission, various techniques have been extensively studied. Amongst others, techniques using trellis coded modulation have been studied. Somehow, these techniques face major problems when used over unknown time-dispersive channels.
In the past, research interests on the adaptive equalization of time-dispersive channels have been mainly centred on uncoded systems:
S. U. H. Qureshi, "Adaptive Equalizer", Proc. of IEEE, Vol. 73, pp. 1349-1387, Sep. 1985;
S. U. H. Qureshi and E. E. Newhall, "An adaptive receiver for data transmission over time-dispersive channels", IEEE Trans. Inform. Theory, Vol IT-19, pp. 448-457, July 1973;
D. D. Falconer and F. R. Magee Jr., "Adaptive channel memory truncation for maximum likelihood sequence estimation", B. S. T. J., Vol. 52, pp. 1541-1562, Nov. 1973;
W. U. Lee and F. S. Hill, Jr., "A maximum likelihood sequence estimator with decision-feedback equalization", IEEE Trans. Comm., Vol. COM-25, pp. 971-979, Sep. 1977;
Y. Gu, T. Le-Ngoc and S. Cheng, "Adaptive decision-feedback equalization with MLSE based on predicted signals", Proc. of ICC'93, Geneva, Switzerland, May 1993; and
J. G. Proakis, "Digital Communications", McGraw-Hill Inc., 1983.
In practice, linear equalizer (LE) and decision-feedback equalizer (DFE) are more often used. DFE has been shown to be superior to LE, especially when the channel has a spectrum null, and almost as effective as the maximum likelihood sequence estimation (MLSE) for the equalization of a large class of time-dispersive channels. However, because of the unavailability of delay-free reliable decisions of received symbols in a system using trellis coded modulation, this conventional adaptive algorithm cannot be directly applied. Several approaches have been proposed to use the conventional DFE for the equalization of unknown time-dispersive channels with TCM signals:
M. V. Eyuboglu, "Detection of coded modulation signals on linear, severely distorted channels using decision-feedback noise prediction with interleaving", IEEE Trans. Commun., Vol. COM-36, pp. 401-409, April 1988;
K. Zhou, J. G. Proakis and F. Ling, "Decision-feedback equalization of time-dispersive channels with coded modulations", IEEE Trans. Commun., Vol. COM-38, pp. 18-24, Jan. 1990;
L. N. Wong and P. J. McLane, "Performance of trellis codes for a class of equalized HF channels", IEEE Trans. Commun., Vol. COM-36, pp. 1330-1336, Dec. 1988; and
P. R. Chevillat and E. Eleftheriou, "Decoding of trellis-encoded signals in the presence of intersymbol interference and noise", IEEE Trans. Commun., Vol. COM-37, pp. 669-676, July 1989.
Eyuboglu proposed a detector using Decision-Feedback Noise Prediction with Interleaving (DFNPI). In the DFNPI, the interleaver/deinterleaver pair rearranges the order of received signals prior to decoding, in a manner that the delayed reliable decisions from TCM detector can be used for feedback.
A similar scheme was presented in Zhou, Proakis and Ling, but a recursive least square algorithm under the minimum mean square error (MMSE) criterion was employed to adjust the DFE instead of the zero-forcing algorithm. Considering the fact that the unpredictable noise in the first received symbol of each interleaving period can considerably degrade the overall system performance, a two-stage structure with reference insertion was studied. Although the DFNPI can approach to the block error rate performance of an ideal DFE, as indicated in: M. V. Eyuboglu and S. U. Qureshi, "Reduced-state sequence estimation for coded modulation on intersymbol interference channels", IEEE J. of Selected Areas on Commun., Vol. SAC-7, pp. 988-995, April 1989, the scheme significantly increases the throughput delay, particularly in the two-stage scheme, and also requires a modification to the transmitter. Apart from these, the reference insertion, introduced to improve the SEP performance, results in a decrease in the effective transmission rate of the system.
It is worth noting that the schemes proposed in Eyuboglu and Zhou do not solve the error propagation problem discussed in S. U. H. Qureshi, "Adaptive Equalizer", Proc. of IEEE, Vol. 73, pp. 1349-1387, Sept. 1985. This is the reason why the probability of block error instead of the SEP is used to measure the performance of the DFNPI.
To avoid the problem of the long throughput delay and the efficiency degradation of the DFNPI, an alternative was investigated by Chevillat and Eleftheriou. In their scheme, the received signals are first pre-filtered by a DFE and then passed through a RSSE detector but the feedback symbols required by the DFE are from the delay-free tentative decision of the RSSE detector. Although superior to the hard-decision maker as in a conventional DFE, the tentative decision is not as reliable as the output to the RSSE detector.
Also known in the art is U.S. Pat. No. 5,031,195 granted on Jul. 9, 1991 which describes a fully adaptive modem receiver using whitening matched filtering for data transmission systems which use trellis-coded modulation and comprises an adaptive whitened-matched filter and a reduced-state trellis decoder. The whitened-matched filter consists of an adaptive linear equalizer with fractionaI-T spaced coefficients, and an adaptive linear predictor. The decoder combines the functions of equalization and trellis-coded modulation decoding. It employs combined intersymbol interference and code states which exploit the set-partitioning structure of the underlying TCM code to provide full or reduced-state information about past ISI terms. The decoder branch metric cancels those ISI terms that are not, or are only partially, represented by the trellis states.
Also known is U.S. Pat. No. 5,303,264 granted on Apr. 12, 1994 which describes an adaptive equalizer using recursive least-squares algorithm and method therefor. This patent describes an arrangement for equalizing waveform distortion caused by multipath fading using a recursive least-squares algorithm into which a forgetting factor is introduced. An incoming signal is applied to an automatic gain controller which generates a first signal whose value varies with a signal-to-noise ratio of the incoming signal. A forgetting factor controller is coupled to the automatic gain controller to receive the first signal and generates the forgetting factor which value varies with the value of the first signal. An adaptive equalizer equalizes the waveform distortion using the forgetting factor according to the recursive least-squares algorithm.
U.S. Pat. No. 5,157,690 granted on Oct. 20, 1992, describes an adaptive convergent decision-feedback equalizer for reducing intersymbol interference in a data communication system. The intersymbol interference is cancelled by generating and subtracting an estimation of the interference from a received signal, which estimation is generated by a N-tap transversal filter in which individual delayed received signals stored in the taps are multiplied by the respective adaptable tap coefficient and summed to form a digital representation of the intersymbol interference present.
In U.S. Pat. No. 5,052,000 granted on Sep. 24, 1991, there is described a technique for improving the operation of decision feedback equalizers in communications systems utilizing error correction. This technique consists of using decision-feedback equalization and error correction coding by means of a plurality of coders and decoders respectively disposed in the transmitter and receiver. The plurality of encoders and decoders is used to interleave the data symbols so that each coder and decoder is operative upon every Mth symbol, where M is the number of coders or decoders. By judiciously choosing M, both the probability of noise impairing the recovery of successive symbols and the error propagation effects inherent in decision-feedback equalizers are reduced.
Also known in the art is U.S. Pat. No. 4,905,254 granted on Feb. 27, 1990 which describes an arrangement for combating intersymbol interference and noise introduced in a data signal transmitted by a dispersive transmission channel. This arrangement comprises an adaptive equalizer with a symbol detector for forming tentative symbol decisions as well as a post-detector for forming final symbol decisions using an auxiliary signal which is derived from the transmitted data signal at the input of the arrangement.
In U.S. Pat. No. 4,953,183 granted on Aug. 28, 1990, there is described an arrangement for combatting intersymbol interference and noise. This arrangement comprises a receive filter, a first decision circuit for forming preliminary symbol decisions in response to the transmitted data signal, a second decision circuit for forming final symbol decisions, means for compensating pre- and post-recursive intersymbol interference, and a combining circuit for forming the input signal for the second decision circuit.
In U.S. Pat. No. 5,263,033, granted on Nov. 16, 1993, there is described a joint data and channel estimation using fast blind trellis search. It consists of finding, for every possible sequence that can be transmitted, the best possible channel fit corresponding to the noisy channel output sequence, by using the least squares procedure. The data and the channel are estimated to be those with the overall best fit.
However, the prior art does not describe a reliable and delay-free decision of received symbols for TCM techniques over time-dispersive channels, and more particularly severely distorted channels. Also, it does not describe a detector avoiding increasing throughput delay and/or decreasing transmission efficiency.